The present invention relates to a static devolatilisation apparatus for devolatilising a viscous liquid comprising a volatile component. The present invention also relates to a process for using said apparatus to devolatilise a viscous liquid comprising a volatile component and the use of said apparatus in the devolatilisation of a viscous liquid.
The devolatilisation of viscous liquids to remove volatile components is of commercial interest. For example, harmful or unwanted volatile components may be removed to improve the purity or other properties of the viscous liquid, or it may be of interest to separate and allow the recovery and potential recycle of volatile components.
In one specific application, polymer devolatilisation is a separation process in which “volatiles” are to be removed from a final polymer, which is in the melt, liquid or solution phase. The “volatiles” to be removed may include solvents, water, residual unreacted monomer(s) (e.g. styrene in the case of polystyrene), by-products, impurities and/or other volatile low molecular weight species such as dimers, trimers, and other oligomeric compounds. Therefore in order to reach a marketable quality, polymers must be degassed at the end of the polymerization step in order to remove such volatiles from the raw resin. This operation is usually achieved by heating the polymer at relatively high temperature (100-350° C.) depending on the polymer, under a pressure which can go from vacuum up to few bars.
The final amount of “volatiles” in the polymer after the devolatilisation is generally required to be quite low, for example, typically between about 100 and about 1000 ppm. Low contents of volatiles are desired to improve the processing and other properties of the polymer. For some specific polymers, removal of toxic monomers and/or solvents may be of importance for Environmental, Health and Safety (EHS) reasons. For example, low levels of volatiles can negatively impact the extrusion, injection molding, or blow molding processing of polymers or lead to formed polymer articles of poor quality.
The separation between polymer and volatile component species in the degassing or devolatilisation process is based on the difference of volatility between those species. The driving force for the devolatilisation is the lower chemical potential in the gas phase than in the polymer. This difference of chemical potential causes the formation of a concentration gradient at the polymer interface resulting in a diffusion flux from the polymer to the gas phase.
The chemical potential of a species in a medium is a function of its concentration in the particular phase and on the temperature. The driving force of the mass transfer can be enhanced by simultaneously increasing the temperature of the polymer and reducing the partial pressure of the species to devolatilise it to the gaseous phase. Those actions are however limited by various factors like the polymer stability and the vacuum system capacity.
The chemical potential allows a determination of the theoretical equilibrium distribution of a solute in a multiphase system, but it does not give any kinetic information on the mass transfer process. In fact, the transfer of a solute between two phases is never instantaneous. The time required to reach the equilibrium state can be significant especially in the case of very viscous liquids such as polymer melts or solutions and/or a small difference of chemical potential between the phases, for example due to a low volatility or a low concentration in the viscous liquid or polymer of the species to devolatilise.
It is often desirable to increase the mass transfer kinetics in order to limit the size of the devolatilisation equipment or the residence time of the polymer under the harsh devolatilisation conditions, which often cause polymer degradation.
Devolatilisation (degassing) typically occurs in two steps. Initially, at high solute concentration, solute bubbles are nucleated, grow in the polymer melt and reach the melt/gas interface where they rupture and solute is released into the gas phase. This initial step known as foam degassing is relatively fast. However, below a certain concentration threshold of solute in the polymer melt, foaming does not take place any more. A stripping agent, which is typically a volatile compound not reacting with the polymer and easy to separate can be admixed to the polymer before the devolatilisation step in order to force foaming and therefore enhance the degassing efficiency. This technique is however not always applicable due to various reasons such as a chemical incompatibility of the stripping agent with the polymer (e.g. causing degradation of the polymer) or an insufficient capacity of the devolatilisation overhead system.
Various types of devolatilizer apparatuses are known. The way the mass transfer between the polymer and gas phases can be enhanced depends on the type of degassing equipment used. Available equipments can be classified into two main families which are the dynamic and the static technologies, each one having its intrinsic drawbacks and advantages. This classification refers to the way the mass transfer is being enhanced.
In “dynamic” equipment the mass transfer is enhanced by the means of moving parts like screws, blades or arms in order to ensure a high rate of surface renewal (higher interfacial concentration gradient) and specific surface creation (more surface through which diffusion can take place). In one case, such apparatuses may have rotating parts, such as in the case of extruders or kneaders, for providing surface renewal, evaporative cooling and efficient mixing to allow an optimal heat and mass transfer. Such a devolatilising extruder is disclosed, for example, in US 2010/0296360 A1. The commercially available dynamic devolatilising or degassing equipment allows a devolatilisation of highly viscous liquids such as polymers, but they have significant drawbacks. Among those, one can note their very high price, complex mechanical construction with tight tolerances, high energy consumption, higher leak rates (thus requiring over-dimensioned vacuum systems), and need for regular maintenance.
In contrast, static devolatilising equipment cannot provide any significant surface renewal, and instead they function mainly based on the creation of a high specific surface by means of dedicated equipment internals. The geometry of those internals is pivotal as they will define the quality of the devolatilising operation in terms of polymer degradation and separation effect. Advantageously such static devolatilizers generally have only pumps, such as discharge pumps for the devolatilised polymer or pumps for heat transfer media, as their moving parts.
A typical internal of a static devolatiliser is a phase separation chamber, often called a “distributor”, having the shape of a tube with a beveled edge. On this edge is mounted a perforated plate through which the polymer melt or solution is freely flowing. The number and size of the perforated plate openings is designed so that the pressure drop through this plate allows the maintenance of a certain level of filling in the distributor. It should not be empty, in which case the polymer residence time would be too low. It should not be full, in which case the pressure in the distributor would increase, preventing an efficient devolatilising of the polymer.
A state of the art static devolatilisation apparatus is disclosed in US 2007/0137488 A1. The drawback of such known static devolatisation apparatuses is that most of the residence time of the polymer takes place in the distributor where the polymer is present as a “bulk” with a very low specific surface exposed to vacuum or reduced pressure. In practice, the polymer is very often foaming in the distributor, filling it completely and therefore limiting the efficiency of the degassing process. After passing through the perforated plate and thereby being dispersed, the polymer flows then straight into the discharge pump and is directly evacuated. The residence time of the dispersed polymer in the devolatilisation vessel is therefore often very limited, typically in the range of seconds for lower viscosity liquids. As a result, the devolatilisation is often insufficient to provide a high quality product having a low concentration of volatiles.
One may try to obtain a higher quality product having a lower concentration of volatiles by increasing the residence time in the static devolatiliser; however, increasing the residence time at elevated temperatures in order to achieve a low content of volatiles typically results in a degradation of thermally-sensitive viscous liquids such as polymer melts and solutions. This is because in conventional static devolatilisers most of the residence time of the polymer takes place in the distributor where only a very low specific surface of the viscous liquid is exposed to vacuum or reduced pressure, and therefore much of the overall residence time in the static devolatiliser is not very effective.
In conclusion, it would be desirable to have a static devolatilision apparatus which allows for a more efficient specific surface creation and a longer residence time of the dispersed viscous liquid (e.g. polymer melt or solution) in the devolatilisation equipment. Such an apparatus would allow for a more efficient devolatilisation for a given residence time or a lower residence time, meaning less viscous liquid (e.g. polymer) degradation, for a given desired volatile component (e.g. residual monomer) concentration in the product.